Flanging and hemming processes for sheet metal parts formed of aluminum alloys and AHSS have been under development for more than a decade. One problem is caused by the reduced ability of aluminum alloys and AHSS to bend around sharp radius.
Several techniques have been proposed to address this problem. For example U.S. Pat. No. 6,257,043 discloses a press hemming process in which the hemming area is compressed while forming the hem. This process was further developed as roll hemming process in U.S. Pat. No. 6,810,707. In the roll hemming process, the local pressure applied to the blank is better controlled than in press hemming process. However, the roll hemming process is substantially slower and is only feasible for low volume or mid-volume applications. Several rolls are required on a hemming tool to provide high volume production. Further development of roll hemming technology is disclosed in U.S. Pat. No. 7,347,072. that proposes making three passes of a hemming roller to form the hem. An alternative technique is disclosed in U.S. Pat. No. 6,928,848 in which the main emphasis is to form a sharp flanging radius during the flanging process by controlling the inner radius of the die instead of the displacement of the punch as proposed in the previously mentioned patents.
One problem not addressed by the above patents is creep of the outer panel during later hemming steps. The “creep” phenomenon occurs due to continued bending of the flange during hemming process. The flanging radius undergoes changes during hemming process that result is a loss of sharpness in the flange after hemming. The continued deformation of the area of flange during following hemming process is caused by the movement of flanged material from outer side of the flange towards inner area of the hem that changes the overall radius of the hem and also adds strain to the previously formed flange area.
This disclosure is directed toward solving the above identified problems and other problems that will be apparent to one of ordinary skill in the art as summarized below.